The invention relates generally to eye testing devices and, more particularly, to eye testing devices for testing visual acuity and including an indicia-bearing chart or panel which provides 100% contrast.
Conventional eye charts typically include graduated rows of dark-colored indicia, such as letters, numerals, or similar characters, painted or otherwise imprinted on a lighter colored panel of cardboard or the like. With age, the individual characters tend to fade at different rates. The characters do not present a 100% contrast when viewed by a patient taking an eye test because even the blackest surface does not absorb all ambient light. The contrast of individual characters can vary considerably depending upon the reflection angle of ambient light shining on the chart. This variation in contrast can affect the accuracy of the visual test, particularly for patients having a reduced ability to differentiate low contrast objects.
Representative prior art construction of other types of eye testing devices and other optical apparatus designed to test visual acuity are disclosed in the following United States Patents:
______________________________________ Bechtold 754,190 March 8, 1904 Rosenbaum 769,805 September 13, 1904 Cross 978,277 December 13, 1910 Freeman 1,591,969 July 13, 1926 Allen 3,469,904 September 30, 1969 ______________________________________
These prior art constructions do not effectively eliminate the above problems. Also, none provide a simple means for conveniently varying the degree of contrast between the characters and the background.